Structural basis for KCTD-mediated rapid desensitization of GABA.sub.B signalling

The GABA.sub.B ([gamma]-aminobutyric acid type B) receptor is one of the principal inhibitory neurotransmitter receptors in the brain, and it signals through heterotrimeric G proteins to activate a variety of effectors, including G-protein-coupled inwardly rectifying potassium channels (GIRKs).sup.1,2. GABA.sub.B-receptor signalling is tightly regulated by auxiliary subunits called KCTDs, which control the kinetics of GIRK activation and desensitization.sup.3-5. However, the mechanistic basis for KCTD modulation of GABA.sub.B signalling remains incompletely understood. Here, using a combination of X-ray crystallography, electron microscopy, and functional and biochemical experiments, we reveal the molecular details of KCTD binding to both GABA.sub.B receptors and G-protein [beta][gamma] subunits. KCTDs associate with the receptor by forming an asymmetric pentameric ring around a region of the receptor carboxy-terminal tail, while a second KCTD domain, H1, engages in a symmetric interaction with five copies of G[beta][gamma] in which the G-protein subunits also interact directly with one another. We further show that KCTD binding to G[beta][gamma] is highly cooperative, defining a model in which KCTD proteins cooperatively strip G proteins from GIRK channels to induce rapid desensitization following receptor activation. These results provide a framework for understanding the molecular basis for the precise temporal control of GABA.sub.B signalling by KCTD proteins. X-ray crystallography, electron microscopy and functional experiments reveal the details of how KCTD proteins interact with GABA.sub.B receptors and desensitize G-protein-coupled inwardly rectifying potassium channels.